A solid-state imaging device that is widely used in a video camera, a digital camera, and the like includes a pixel portion that includes a plurality of photodiodes formed into a lattice shape on a semiconductor substrate, a vertical transfer register that is connected to the pixel portion, and a horizontal transfer register that is connected to the vertical transfer register.
In the solid-state imaging device, a photodiode converts received light into electron. The electrons as a result of the conversion are sequentially transferred by the vertical transfer register and the horizontal transfer register, and finally a desired image is output by performing charge-voltage conversion.
The vertical transfer register or the horizontal transfer register of the solid-state imaging device has the following configuration.
For example, an n-type channel region is formed in a surface of a p-type silicon substrate. In a surface of the channel region, a plurality of p-type transfer barrier portions are provided in a row with a space between one another by ion implantation of boron or the like. A plurality of transfer electrodes are formed in a row with a space between one another on the silicon substrate in which the channel region and the transfer barrier portions are formed, and an insulating film is interposed between the transfer electrodes and the silicon substrate. Each of the transfer electrodes is formed in a region extending from one end of the transfer barrier portion over the transfer barrier portion toward a charge transfer direction to the channel region.
The solid-state imaging device transfers the charge by alternately applying a voltage to the transfer electrodes formed in a row to control a potential in the silicon substrate.
In producing the solid-state imaging device, after the transfer barrier portions are formed by subjecting the surface of the channel region to ion implantation, the transfer electrodes are formed by, for example, a lift-off method. However, the transfer barrier portions cannot be visually recognized even if they are formed. Accordingly, it is difficult to overlay the transfer electrodes on the transfer barrier portions.
In this regard, conventionally in producing a solid-state imaging device, an alignment mark that serves as a reference for alignment is formed by etching outside a device region on the silicon substrate, and then the above-described process is performed using the alignment mark as a reference.
As another producing method of the solid-state imaging device, there is known a method for forming the transfer barrier portions and the alignment mark through a single resist layer. In the method, first a resist layer is formed on a silicon substrate. The resist layer has an opening for forming an alignment mark and thin-film portions for forming transfer barrier portions. Then, the silicon substrate is subjected to dry etching through the resist layer, thereby forming the alignment mark. Next, the ion implantation is performed through the resist layer.
According to the method, because the alignment mark and the transfer barrier portions are formed using the single resist layer, the process of forming only the alignment mark can be eliminated. In addition, because the resist layer for forming the transfer electrodes can directly be aligned with the alignment mark as a reference, the transfer electrodes can accurately be formed in desired positions.
However, in the method, because the dry etching is performed first to form the alignment mark, the thin-film portions of the resist layer are also etched by the dry etching. Therefore, in the subsequent ion implantation process, the ion implantation is performed through the thin-film portions having a thickness smaller than a designed thickness. Therefore, it is disadvantageously difficult to form the transfer barrier portions to have a given depth with high accuracy.